THIS INVENTION relates to improvements in hydraulic mechanisms such as fasteners or nuts and in particular it relates to improvements in the seals to such mechanisms and fasteners.
Hydraulic systems such as nuts and like type fasteners are known, The nuts provide a means by which a stud or bolt can be tensioned on being engaged by the nut, which nut is then hydraulically actuated to apply a tensile force in the stud or bolt. The nuts often operate under extremes of pressure and temperature.
Hydraulic nuts or like type fasteners are typically pre-tensioned mechanically, after which a source of hydraulic pressure is applied to a chamber within the structure to generate an hydraulic force which applies an axial tensile force to a stud or nut engaged by the fastener. A locking collar may be used to retain that tension after release of the pressure from said chamber.
The magnitude of the added tensile force is dependent upon the operative surface area of the hydraulic chamber in the nut and that pressure which is introduced into the chamber and acts upon it. Often the available operative surface area of the hydraulic chamber is limited by the juxtaposition of adjoining features and the necessary thickness of its internal structure to withstand stresses generated by the introduced fluid pressure. In such cases a stacked array of chambers may be utilised (see U.S. Pat No.4,326,826xe2x80x94Bunyan)
The expansion chambers of the above style nuts need to be sealed. In some assemblies pressurising fluid is retained within a bladder (see U.S. Pat. No. 4,854,798xe2x80x94Snyder). Most often seals are annular rings (see U.S. Pat. No. 4,074,923xe2x80x94Lathara).
Seals for use with high pressure hydraulic devices are typically made of elastomeric material such as nitrite rubber or polyurethane. The means by which these seal against the passage of fluid pressure can be divided into two distinct parts or mechanisms, described herein as primary and secondary mechanisms. The primary mechanism of sealing acts during the initial application of fluid pressure, and simply blocks the passage of fluid, allowing internal pressure to rise. As this pressure increases, the elastomeric seal is is deformed and is forced to a position where the seal bridges the gap which is to be sealed, herein referred to as the extrusion gap, to establish a secondary seal.
Prior art document U.S. Pat. No. 5,468,106 Percival-Smith shows seals, supposedly for Operation at higher temperatures than is achieved by conventional seals. Its seals are integrated with components of the hydraulic assembly and these are therefore non replaceable. Sealing is accomplished by deflecting a thin edge of a component to bridge the extrusion gap. A very rapid surge of viscous fluid is required to displace this edge.
It is typical of hydraulically activated piston and cylinder arrangements that as operating pressures increase, the cylinder walls are expanded radially, causing a proportional increase in the extrusion gap between piston and cylinder. This is particularly a feature of the above stacked configuration (Bunyan). Stacking is effected because of limitations to radial dimensions and the walls of such nuts are limited as to thickness. The walls of these nuts are particularly subject to increase in size of the extrusion gap as pressures are increased. There is a need for a seal which is re-usably operative in these and other systems, and at high temperature and pressures. The design of Percival-smith does not achieve good low pressure performance and it does not offer a useful re-usable seal.
A limiting feature in the operation of hydraulic nuts is the effectiveness of their seals. Factors such as high pressures, high temperatures, and service life under adverse conditions curtail their application and effectiveness. If these factors become extreme, either singly or in combination, then materials which are commonly used as sealing agents will fail. A failure mode is the flow or movement of the material of the seal into the extrusion gap under pressure and/or temperature. At this point the seal can be lost.
It is an object of the present invention to provide an hydraulic assembly, such as a fastener, with improved sealing characteristics able to tolerate more extreme factors such as high pressures and high temperatures, achieving a more extended service life under such adverse conditions.
The invention achieves its object in the provision of a seal for an hydraulic assembly wherein hydraulic fluid is to be contained in a working chamber formed between at least two parts of the assembly operating in at least two modes, a primary sealing mode and a secondary sealing mode, the primary sealing mode being operative to contain hydraulic fluid to a first pressure level, and the secondary sealing mode being operative above said first pressure level with elastic deformation of a metallic part of the seal to closely associate the seal material with any gap between the at least two parts of the assembly.
The seal of the invention is particularly suited to applications where there is a substantial extrusion gap increase with applied pressure. The seal of the invention travels with the outer wall. It does not separate from it. The shoulder of the seal maintains a sliding contact with the radially expanding component of the assembly.
In a preferred embodiment the seal may be one which provides secondary sealing off sloped shoulders, a resolved force along the slope creating the secondary mode of sealing. The slope and material may be matched to achieve secondary sealing without the down force causing the seal base to adhere under friction to the ramp before the seal can react. The specific nature of a desirable seal is a combination of factors involving angle of slope, choice of material, target temperatures, and target pressures, such that no one design is forced for any particular application.
In a further and different embodiment the seal may be one wherein it is a pressed, cup shaped form, with flanges to form primary seats, and reactive to pressure to form a secondary seal. The cup form can be filled with materials with resistance to crushing such as ceramics.
Ideally the seal of the invention is one wherein dissimilar metals are chosen for the seal and the hydraulic assembly components being sealed, to avoid fusing at extreme pressures.
It will be evident that an adaptation of the hydraulic assembly, mechanism, or fastener, at the point where seals are fitted to the expansion chamber, will be made to accommodate the selected shape of the seal. A sloped shoulder will be effective in promotion of the secondary mode of sealing. A slope sufficient to enhance operation without inhibiting interaction between the respective surfaces of shoulder and seal is desirable. It will be clear to those skilled in the art that surface preparation will be important to establishing what level of interaction might arise between two such surfaces, in addition to choice of material, and geometry of the interacting surfaces. Various combinations of the aforesaid factors will provide increased radial thrust, to improve secondary sealing, in any given assembly. A desired thrust is resolved from that action of hydraulic forces directly on an angled base of the seal. Ramp angle for the base is ideally calculated to both prevent frictional adhesion of the seal to the romp before it can react, or wedging of the seal resulting impediment to free movement of the sliding components.
A number of seal constructions will be seen herein to be possible. The seal may be a one piece all metal seal, having both primary and secondary sealing functions. Alternately a one piece metal seal might be a pressed, cup shaped seal with flanges to form primary seals, reactive to pressure to form a secondary seal. Other materials with resistance to crushing might be used such as ceramics.
The yield strength of the materials used is desirably in excess of the sum of pressure and radial loads induced by operation, for repeated operation. Otherwise the seal may be limited to a single use. Below the yield strength the rings will possess elastic properties, permitting re-use for a longer service life and durability.
Whilst the invention is described herein with particular reference to hydraulic nuts, the seals of the invention have application in any hydraulic assembly where there is a working chamber to be operated, in use, under pressure of an hydraulic fluid, and it is desired to achieve a higher level of performance, at higher temperatures and pressures.